Audio and video disc playback systems of the type using a disc having recorded information signals encoded into a fine spiral groove typically utilize a stylus to read the encoded information signals. Included in such systems are the capacitive playback systems, wherein the stylus dielectric material is coated on at least one surface with a conductive metal layer which interacts with the conductive disc to read the informtion encoded thereon. An example of such a system is disclosed in U.S. Pat. No. 3,842,194 issued Oct. 15, 1974 to Clemens. Clemens discloses a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc. For example, groove widths of about 2.5 micrometers and groove depths of about 1.0 micrometer may be used. During playback a pickup stylus having a shoe width of 2.0 micrometers and a thin conductive electrode formed thereon, for example about 0.2 micrometer thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the disc surface are sensed to recover the pre-recorded information. In systems of the above type, the use of a relatively fine record groove and the requirements for a groove engaging pickup stylus result in a stylus tip which is extremely small. Further, the length of the stylus shoe is critical because if it is too short the stylus may damage the disc due to excessive pressure. If the stylus shoe is too long, on the other hand, stylus life is reduced and electrical performance is degraded.
During the manufacturing of such styli frequent inspection of the tip is therefore necessary. This inspection is usually done by examination through a high power optical microscope having, for example 1000.times. to 1500.times. magnification. The stylus tip is centered in the field of view of the microscope by an operator and the shoe of the stylus is visually compared to a reticle. The length of the shoe is estimated and a determination made as to the need for further lapping of the stylus shoe. The present lapping process used in the industry produces somewhat inconsistent results from one stylus to another. Since excessive lapping results in a defective stylus, frequent and accurate measuring of the shoe length is essential.
In a mass production environment a system that requires an operator to look into a microscope and make a dimensional measurement near the resolution limit of the microscope is unattractive for many reasons. Operator fatigue is likely to occur and occasionally an erroneous measurement will be made.
What is needed is an apparatus that is easily and quickly manipulated by a relatively unskilled operator to automatically make the desired measurement.